Master-oscillator power-amplifier (MOPA) excimer or molecular fluorine laser system with long optics lifetime

ABSTRACT

The lifetime of optical components used in deep-UV (DUV) excimer laser systems, including systems in a MOPA configuration, can be increased by reducing the intensity of pulses incident upon these components. In one approach, an output pulse can be “stretched” in order to reduce the peak power of the pulse. A pulse stretching component can be used, which can be mounted outside the laser enclosure with a horizontal beam path in order to provide a delay line with a minimum impact on the laser system footprint. The horizontal beam path also can minimize the number of optical components in the arm containing the high power beam. A beamsplitting prism can be used with the delay line to avoid the rapid degradation of coatings otherwise exposed to intense UV beams. The prism can expand the beam in the delay line in order to minimize beam intensity and losses due to reflection.

CLAIM OF PRIORITY

This application is a divisional of U.S. patent application Ser. No.10/881,103, filed Jun. 30, 2004, which in turn claims priority to U.S.Provisional Application No. 60/484,046, filed Jul. 1, 2003, as well asU.S. Provisional Application No. 60/484,929, filed Jul. 3, 2003, each ofwhich is hereby incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a Master-Oscillator Power-Amplifier(MOPA) excimer or molecular fluorine laser system, particularly a MOPAsystem having optics with a relatively long lifetime.

BACKGROUND

Semiconductor manufacturers are currently using deep ultraviolet (DUV)lithography tools based on KrF-excimer laser systems, operating atwavelengths around 248 nm, as well as ArF-excimer laser systems, whichoperate at around 193 nm. Vacuum UV (VUV) tools are based on F₂-lasersystems operating at around 157 nm. These relatively short wavelengthsare advantageous for photolithography applications because the criticaldimension, which represents the smallest resolvable feature size thatcan be produced photolithographically, is proportional to the wavelengthused to produce that feature. The use of smaller wavelengths can providefor the manufacture of smaller and faster microprocessors, as well aslarger capacity DRAMs, in a smaller package. In addition to havingsmaller wavelengths, such lasers have a relatively high photon energy(i.e., 7.9 eV) which is readily absorbed by high band gap materials suchas quartz, synthetic quartz (SiO₂), Teflon (PTFE), and silicone, amongothers. This absorption leads to excimer and molecular fluorine lasershaving even greater potential in a wide variety of materials processingapplications. Excimer and molecular fluorine lasers having higherenergy, stability, and efficiency are being developed as lithographicexposure tools for producing very small structures as chip manufacturingproceeds into the 0.18 micron regime and beyond. The desire for suchsubmicron features comes with a price, however, as there is a need forimproved processing equipment capable of consistently and reliablygenerating such features. Further, as excimer laser systems are the nextgeneration to be used for micro-lithography applications, the demand ofsemiconductor manufacturers for powers of 40 W or more to supportthroughput requirements leads to further complexity and expense.

In laser systems used for photolithography applications, for example, itwould be desirable to move toward higher repetition rates, increasedenergy stability and dose control, increased system uptime, narroweroutput emission bandwidths, improved wavelength and bandwidth accuracy,and improved compatibility with stepper/scanner imaging systems. It alsowould be desirable to provide lithography light sources that deliverhigh spectral purity and extreme power, but that also deliver a low costchip production. Requirements of semiconductor manufacturers for higherpower and tighter bandwidth can place excessive, and often competing,demands on current single-chamber-based light sources. Many of theseobstacles are overcome by taking advantage of adual-gas-discharge-chamber technology referred to herein as MOPA (MasterOscillator—Power Amplifier) technology. MOPA technology can be used toseparate the bandwidth and power generators of a laser system, as wellas to control each gas discharge chamber separately, such that both therequired bandwidth and pulse energy parameters can be optimized. Using amaster oscillator (MO), for example, an extremely tight spectrum can begenerated for high-numerical-aperture lenses at low pulse energy. Apower amplifier (PA), for example, can be used to intensify the light,in order to deliver the power levels necessary for the high throughputdesired by the chip manufacturers. The MOPA concept can be used with anyappropriate laser, such as KrF, ArF, and F₂-based lasers.

Components of a MOPA laser system can include those discussed in U.S.Pat. No. 6,577,663, hereby incorporated herein by reference, whichdiscloses a molecular fluorine (F₂) laser system including a seedoscillator (or master oscillator) and power amplifier. The seedoscillator comprises a laser tube including multiple electrodes therein,which are connected to a discharge circuit. Seed radiation canalternatively be provided by an excimer lamp maintained at low pressure.The laser tube is part of an optical resonator for generating a laserbeam including a first line of multiple characteristic emission linesaround 157 nm. The laser tube can be filled with a gas mixture includingmolecular fluorine and a buffer gas. The gas mixture can be at apressure below that which results in the generation of a laser emission,including the first line around 157 nm having a natural line width ofless than 0.5 pm, without an additional line-narrowing optical componentfor narrowing the first line. The power amplifier increases the power ofthe beam emitted by the seed oscillator to a desired power forapplications processing. A power amplifier (PA) typically includes adischarge chamber filled with a laser gas, such as a gas includingmolecular fluorine, and a buffer gas. Electrodes positioned in thedischarge chamber are connected to a discharge circuit, such as anelectrical delay circuit, for energizing the molecular fluorine in thechamber. The discharge of the PA can be timed to be at, or near, amaximum in discharge current when a pulse from the master oscillator(MO) reaches the amplifier discharge chamber. Various line-narrowingoptics can be used, such as may include one or more tuned or tuneableetalons.

A major limitation to deep-UV (DUV) excimer laser systems is thelifetime of the optical components used therein. Especially at the levelof output power on the order of tens of Watts, fast decay of opticalcomponents increases downtime as well as operational costs forapplications such as microlithography. While implementation of the MOPAconcept can increase the output power of the laser system, one offundamental limitations of existing MOPA systems is this relativelyshort optics lifetime. For example, one of the principal factorslimiting the optics lifetime in a MOPA system is the fast degradation ofthe output window of the amplifier discharge chamber, as this opticalcomponent is typically at the point where the power is at the highestlevel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an optical layout that can be used inaccordance with one embodiment of the present invention.

FIG. 2 shows a (a) side view of a MOPA laser system including a pulseextender module and a (b) corresponding top view of the pulse extendermodule top that can be used in accordance with one embodiment of thepresent invention.

FIG. 3(a) shows details of a beamsplitter arrangement that can be usedin accordance with one embodiment of the present invention.

FIG. 3(b) shows a detailed view of beam portions at the beamsplitter ofFIG. 3(a).

FIG. 3(c) shows an exemplary pulse stretcher output waveform.

FIG. 3(d) shows details of another beamsplitter arrangement that can beused in accordance with one embodiment of the present invention.

FIG. 3(e) shows details of another beamsplitter arrangement that can beused in accordance with one embodiment of the present invention.

FIG. 4 shows details of a beam path that can be used in the amplifier ofFIG. 1(b).

FIGS. 5(a) and 5(b) show details of a spatial filter that can be used inaccordance with various embodiments of the present invention.

FIG. 6 shows details of another beam path that can be used in theamplifier of FIG. 1(b).

FIG. 7 shows details of (a) a coated plate beam splitter and a (b) prismbeam splitter that can be used with the arrangement of FIG. 6.

FIGS. 8(a)-8(c) show various beam expanding approaches that can be usedin accordance with various embodiments of the present invention.

FIG. 9 shows an extended output window position that can be used inaccordance with various embodiments of the present invention.

FIG. 10 shows an overall laser system showing components that can beused with a system in accordance with various embodiments of the presentinvention.

DETAILED DESCRIPTION

Systems and methods in accordance with various embodiments of thepresent invention can employ any of a number of optical layouts andapproaches in order to increase the lifetime of various system optics.These approaches can include, for example, reducing the average powerdensity and peak intensity of a beam (or optical pulse) that passesthrough these optical components. This reduction can be significant forthose optical components, as a high-power beam output from an amplifierin a MOPA system such as that shown in FIG. 1 can reach a power level onthe order of 100 W, which can lead to significant degradation of theoptical components.

FIG. 1 shows a top view block diagram of an exemplary optical layout 100that can be used with a MOPA system in accordance with one embodiment ofthe present invention. Various other MOPA configurations, as well asdetail regarding the workings of MOPA systems, are disclosed in pendingU.S. patent application Ser. No. 10/696,979, filed Oct. 30, 2003, whichis hereby incorporated herein by reference.

In the generalized configuration 100 of FIG. 1, a master oscillator (MO)102 includes a first discharge chamber having disposed therein a pair ofelectrodes 112 on either side of a beam path through the MO forgenerating an optical pulse. The MO can include a line-narrowing opticsmodule 110 for narrowing an oscillator beam (or optical pulse) in thedischarge chamber, and an outcoupler module 114 for outcoupling thebeam. Although not shown, it should be recognized by one of ordinaryskill in the art that various other elements can be included in theoscillator which are not shown, such as output beam diagnostic tools,circuits for forming a discharge pulse, and electronic controls. Many ofthese components will be discussed with respect to FIG. 10.

A power amplifier (PA) 104 can be positioned along the beam path at adistance from the MO, such as a distance comparable to the pulse lengthin free space, or at least half of the pulse length. The beam (or pulse)can pass directly to the amplifier, or can be redirected or otherwiseaffected by any of a number of optical elements positioned between theoscillator and amplifier. Further, any of a number of beam expandingelements, such as a prism and a focusing lens, can be used to expand thebeam before the beam passes through the amplifier. Expanding the beamcan reduce the intensity of the beam, while allowing the beam to utilizemore of the area between the electrodes in the power amplifier, in orderto maximize the gain obtained through amplification. The beam expandingoptics can produce a diverging or non-diverging beam as known in theart. Beam bending optics also can be used to alter the path of the beam,such as to direct the beam to the amplifier and/or increase the pathlength between the oscillator and amplifier.

The intensity of the beam between the oscillator and the amplifier isnot very high, typically having pulse energy on the order of a fewmilli-Joules or less. Optical elements such as prisms or mirrorstherefore can be used effectively. The separation between the MO and PAcan cause any amplified spontaneous emission (ASE) from the PA to bedelayed with respect to the initial stages of pulse formation in the MO,such that pulse formation is not disturbed. A spatial filter 116 can belocated along the beam path between the MO and the PA, which can serveto further de-couple the MO and the PA, and which can modify the size ofbeam size as described elsewhere herein. The PA can include a dischargechamber containing at least one pair of electrodes 118 on either side ofthe beam path. Once the beam makes a first pass through the PA, the beamcan exit as an output beam and/or can pass through a ring cavity for asecond pass through the PA 104, as described elsewhere herein.

At the output of the amplifier chamber, the average power and intensityof the beam are typically the highest in the system. The first opticalcomponent that the amplified beam encounters is generally the outputwindow that seals the amplifier chamber. This window can be anun-coated, plano-parallel window made of CaF₂ or MgF₂, for example. Inorder to reduce the intensity of the beam per unit area of the window,the window can be tilted with respect to the beam (as shown in FIG. 4).The intensity of the beam at the window surface can be reduced by afactor of approximately 1.5 when such a window is placed at the Brewsterangle relative to the beam.

After exiting the power amplifier, the beam can pass through at leastone beam bending and/or expanding optical component 106. After passingthrough the beam bending/expanding optic(s) 106, the beam can passthrough a pulse stretcher component 108, which can effectively stretchthe length of the output pulse as described below. In certainembodiments, it can be advantageous to increase the output pulse lengthin order to, for example, reduce the peak power in the opticalcomponents of the stepper. It also can be practical to use at least oneof the beam bending/expanding optics as part of a delay line of thepulse stretcher, where possible. As known in the art, a delay line caninclude any of a number of reflective, refractive, or transmissiveoptical elements capable of directing a beam along an optical path of apredetermined length, such that the beam exits the delay line after apredetermined amount of time has passed from the entrance of the beaminto the delay line. A delay line typically includes a number of turningmirrors for directing the beam.

Several beam bending and/or beam expanding optics can be used to directand expand the beam after passing from the discharge chamber of thepower amplifier. A beam bender in one embodiment includes four prismsused to fold the beam, simultaneously changing the polarization whennecessary for the accompanying pulse stretcher. For instance, a pulsestretcher can be placed within the body of the laser in a vertical planein order to allow utilize the footprint of the laser with a minimalnumber of beam folding optics. In order to provide an output beam in thehorizontal plane, the beam can be further bent using another beambender, which can include a pair of prisms, for example.

After passing through the beam bending and/or expanding optics, the beamcan pass through a pulse stretching component 108. A pulse stretchingcomponent can be used to obtain a pulse length of around 200 ns asdesired by the Lithography industry, for example, as an excimer lasertypically has a pulse length around 20-35 ns. Such a pulse length can beobtained by stretching the pulse with a pulse extending/stretchingcomponent, or “pulse stretcher.” The length of the output pulse afterthe pulse stretcher can depend on the length of the delay line insidethe pulse stretcher. For optimum performance in one embodiment, thedelay within the delay line should be as long as the pulse length of theinput pulse. For an input pulse of 30 ns, then, a delay line length ofabout 30 ns is used which leads to an optical path length of about 10 m.It can be desirable for the length of the delay line to be substantiallyequal to the length of the input pulse, as a shorter delay time canresult in overlap between the reflected and delayed pulses. Further, alonger delay time can lead to gaps between the reflected and delayedpulses.

Arranging the delay line vertically is feasible, as discussed above, butthe beam may need to be folded many times in order to obtain the desiredoptical path length. This is due at least in part to the limited spacein vertical direction of the laser device. Typically, a vertical spaceof about 1.5 m is available inside the laser enclosure, with anavailable width of less than one meter. For typical pulses on the orderof 20-30 ns, the optimal delay line length then is on the order of 6-10m. In order to obtain a path length of 10 m in such an enclosure thebeam would have to be folded 6-8 times. As each mirror can introduceadditional losses, it can be preferable to minimize the number ofmirrors.

One embodiment that can provide the necessary path length whileminimizing the number of additional mirrors is shown in FIGS. 2(a) and2(b). In this exemplary system 200, a laser housing 202 is shown tocontain the master oscillator 206 and power amplifier 208, as well asany turning mirrors or other components that are typically included insuch a laser housing. The housing can be made out of any appropriatematerial, such as for example sheet metal or plastic. The housing alsocan have at least one output window 228 in an end panel of the housingfor transmitting a generated laser beam as an output pulse. The endpanels (not shown) of the laser housing 202 in this embodiment are about1.0 m wide and 1.5 m high. This exemplary laser housing also has upperand lower horizontal panels 224 forming a top and bottom to the housing,with each horizontal panel being about 1.0 m wide and 3.0 m long. Thelaser housing also has vertical side panels (not shown but parallel tothe plane of the Figure in FIG. 2(a)) forming sides to the housing, witheach vertical panel being about 1.5 m high and 3.0 m long. A horizontalpulse extending module 204 can be placed adjacent to one of thehorizontal panels 224, such as a distance above and parallel to theupper horizontal panel 224 of the laser enclosure 202. Placing the pulseextender outside the housing but parallel to one of the horizontalpanels allows for use of an amount of space approximately equal to thefootprint of the laser enclosure 202, which can use approximately theentire 3 m length of the housing as opposed to the 1.5 m height forvertical extenders, whereby folding the beam three to four times can besufficient to obtain the desired path length. A horizontal pulseextender also can be more efficient and much simpler than the verticalcounterpart, and can be replaced easily without interfering with thealignment or the laser itself.

Different pulse extenders can be built, each of which can stretch thepulse by a different factor based on the optical path. In a simple case,a pulse extender can provide a stretching factor of about 3.0, whereinthe stretcher includes a beam splitter and three folding/imagingmirrors. More sophisticated models can include, for example, a “striped”or graded reflectivity beam splitter capable of stretching the pulse bya factor of about 4-10, or a sequential pulse extender where twoindividual extenders with different optical path lengths are arranged inseries. Striped beam splitters are described, for example, in U.S. Pat.No. 7,035,012, entitled “Optical Pulse Duration Extender,” which ishereby incorporated herein by reference. The pulse extending module canbe contained within a housing of a comparable footprint and/or materialto that of the laser housing.

As seen in the side view of FIG. 2(a), a horizontal beam (or pulse)exiting the oscillator chamber 206 and passing through the amplifierchamber 208 can be folded vertically by a mirror 210 and/or at least oneprism. The folded beam can leave through one of the horizontal housingpanels 224 of the laser enclosure 202 and enter the pulse extendermodule 204. There, the beam can be folded back onto the horizontal axisby a folding mirror 214, and can directed to a beam splitting element216, such as a dielectrically-coated plate or a prism beamsplitter asdiscussed below. The beamsplitter can cause a portion of the beam to bereflected to a folding mirror 222 that directs the beam out of the pulseextender module and back into the laser enclosure 202. The remainingportion of the beam can be directed by the beamsplitter through a delayline of the pulse extender, here including relay imaging mirrors 218 and220. After passing through the delay line, at least a portion of thedelayed beam can be directed to folding mirror 222 and directed backinto the laser enclosure 202. At the desired exit height, for example,the beam can be reflected by a mirror 212 and/or at least one prism ontothe horizontal axis. Additional mirrors, prisms, or other steeringoptics can be used to steer the output beam, either before or afterexiting the laser enclosure 202 through an output window 228. Extendingthe pulse can reduce the intensity of the pulse, such that the life ofthe output window is extended.

The path of the beam inside the pulse extender can include anyappropriate path, such as the example shown in the top view of FIG.2(b). Here, after mirror 214 folds the beam into the horizontal plane, atriangular beam path is created by beamsplitter 216 and folding elements218 and 220. It should be understood that any of a number of paths, suchas rectangular or irregular beam paths, can be created using any numberof turning elements as known in the art, in order to obtain the properpath length in the available amount of space, and should not be limitedto the examples discussed herein. After the beam has passed through theoptical delay line of the pulse extender module 204, another foldingoptical element (or the same as the input element) can direct the beamback into the laser enclosure for output as an output pulse. Redirectingthe beam through the original output window of the laser enclosureprevents the need to realign the optics or adjust the footprint of theoverall system.

In another embodiment, a beam exiting the discharge chamber of the poweramplifier, which may also pass through a set of beam bending and/orexpanding optics, can pass through a pulse extending (or pulsestretching) component 300 such as that shown in FIG. 3(a). This pulsestretching component can consist of a beamsplitter 302 and at least twofolding and imaging mirrors 304, 306. While this pulse stretching modulecan be used as a stand-alone module, such as in a vertical orientation,it should be understood that the approach also could be used with thehorizontal pulse extender module of FIG. 2(a). The beamsplitter 302 hasa substantially planar first surface that can reflect a portion 308 ofthe pulse energy towards the output. If an optical pulse is incident onthe planar first surface at a shallow angle, such as about 9° relativeto the plane of the surface, then the reflected portion will bereflected toward the output window at an angle of about 9° relative tothe surface. At least one beam bending element can be used to direct thereflected portion to the output window where necessary. In anotherembodiment, any necessary beam bending can be done before thebeamsplitter, with the beamsplitter being oriented such that thereflected portion is reflected directly to the output window along theoutput beam path.

The portion of the optical pulse that is not reflected by the firstsurface will be substantially transmitted through the beamsplittingelement as a transmitted portion 310. If the beamsplitting element was aplanar plate as in existing systems, the beam expansion in thebeamsplitter would be compressed away upon exit of the beamsplitter.Since the present embodiment uses a beamsplitting prism 302, shown inmore detail in FIG. 3(b), the angled second surface 322 opposite theplanar first surface 320 allows the transmitted portion to be expandedwhen exiting the beamsplitting prism. The amount of expansion can becontrolled by the angle of the second surface 322, relative to theplanar first surface 320 and the angle of incidence of the optical pulseon the first surface. For instance, the second surface 322 of thebeamsplitter 302 is shown to form an apex angle θ with the third surfaceportion 324. Each of the second and third surfaces is angled withrespect to the first surface, such as an angle of about 7°-9° in FIGS.3(a) and 3(b). In one example, the prism is an equilateral prism with anapex angle of about 165°. For this example, the preferred range isbetween about 155° and 175°. The angle causes the beam to be expanded byabout a factor of 3 when exiting the second surface of the beamsplittingprism. The amount of expansion can be selected to obtain a desiredreduction in intensity of the beam, in order to extend the lifetime ofthe system optics. In FIG. 3(a), the expanded beam passes through adelay line and back to the third surface 324. Since the second surfaceand third surfaces are similarly angled with respect to the firstsurface, but in the opposite direction, the expanded beam will becompressed when passing back through the beamsplitting prism, such thatthe transmitted beam exiting the first surface will be approximately thesame width as the original beam that was incident on the first surface.Further, the path of the transmitted beam will be substantially the sameas the path of the reflected beam. This allows both the initial anddelayed beam portions to follow the same output path without anyadditional optical elements. In one embodiment, the delay line isconfigured such that the transmitted portion intersects the second andthird surfaces of the prism at similar angles in order for the initialand delayed portions to follow the same exit path.

The transmitted portion 310 of the pulse is delayed with respect to thereflected portion 308 by an amount that equals the time for thetransmitted pulse to follow one round-trip through the delay line formedin part by mirrors 304, 306. While at least a portion of the pulse canbe added to the output upon transmission through the beamsplitter 302, aportion of the transmitted beam can be reflected by the first surfaceand directed back through the delay line. A reflected portion of thedelayed pulse then can be further delayed in subsequent round-tripsthrough the delay line. The output of the pulse stretching component inthis embodiment then is a series of pulses spaced apart by a timeinterval defined by the delay line, with a correspondingly diminishedenergy per pulse. The resulting “Time-Integral Square” (TIS) length ofthe output pulse is increased, as compared to the input pulse. Theresulting central waveform of such an output pulse can be as shown inFIG. 3(c). The solid line 350 in the plot represents the overallintensity of the output pulse waveform, while the dashed lines 352, 354represent the individual intensities of output pulses from individualround trips in the delay line. The separation between peaks of theoutput pulses due to the delay line can be seen. The amount of delay inthe delay line can be, for example, approximately equal to the incidentpulse length at the 1/e² level. A greater delay typically will notprovide a significant increase of the TIS pulse length, but typicallywill require more space. At the same time, a shorter delay can decreasethe output TIS. The transmission/reflection ratio of the beamsplittercan be adjusted in such a way that the first two output pulses of thewaveform are of approximately equal energy, thereby providing anearly-maximum TIS pulse length-stretching ratio. In one example, atransmission rate of about 65% has been shown to be effective, assuminglosses of about 17% in the delay line.

As shown in FIGS. 3(a) and 3(b), the beamsplitting element 302 can be anequilateral prism, where the second and third surfaces 322, 324 can havean apex angle θ that, for at least one embodiment, is optimally between90° and 180°, or between 90° and 165°. Smaller apex angles can increasethe expansion of the beam transmitted from the second surface 322 of thebeamsplitter, as described below, while larger angles can reduce lossesdue to reflection and can allow for a thinner overall prism. It can beadvantageous to use a prism that is as thin as possible, as certainmaterials are not completely uniform and can cause artifacts and/orlosses in the beam. The apex angle θ of the embodiment shown in FIG.3(b) is approximately 165°. The apex angle can determine the expansionof the transmitted beam in the delay line, and also can affect the shapeand/or footprint of the delay line by affecting the angle at which thetransmitted beam exits the beamsplitter. The selection of the apex anglecan strike a balance between beam expansion and intensity losses due toreflection at the second surface of the prism. An apex angle thatresults in the transmitted beam exiting the second surface in adirection normal to the second surface can provide the greatest amountof beam expansion, but also can experience the highest amount ofintensity loss due to reflectance from the second surface, such as aloss on the order of about 4%. It therefore can be necessary to selectthe apex angle to balance the amount of possible beam expansion with theamount of acceptable intensity loss. In the embodiment of FIG. 3(b), itwas found that an acceptable balance was obtained with an apex angle ofabout 165°. With an apex angle of 165°, an incidence angle of 81°relative to normal results in a beam expansion of a factor of 3.2, withminimal losses due to the beam striking the second surface at theBrewster angle. If the apex angle were 98°, by comparison, the beamexpansion would be about a factor of 4.8, with losses at the secondsurface on the order of 4%.

A beamsplitting prism 302 can be made of any appropriate material, suchas CaF₂, excimer-grade fused silica, magnesium fluoride, or sapphire,for example. Since the angle of the prism relative to the incident beamis used to control the reflectance, and the apex angle of the prism isused to control the bending and expansion of the transmitted beamportion, no partially reflective coating or dielectric coating isrequired on the first, second, or third surfaces. The lack of such acoating can provide for a longer lifetime than is observed for coatedbeamsplitters. Further, use of such a prism allows the beam to be spreadover a larger surface area than other beam splitters due to the range ofapex angles, allowing for a lower intensity per unit area and acorresponding increase in optic lifetime.

As shown in the example of FIG. 3(b), an incident beam (or pulse) canintercept a first surface 320 of the beamsplitter at a predeterminedangle α, which in this case is at an angle of approximately 8.4° suchthat the incident ray strikes the first surface at about 81.6° fromnormal to the surface 320. The angle at which the incident beam strikesthe first surface 320 of the beamsplitter can be selected in order toobtain the desired reflectance from the first surface. Using such smallangles allows reflectances on the order of 25%-35% to be obtained, suchthat there is no need for a reflective, or partially reflective, coatingon the first surface. The reflectivity R_(p) of the uncoated surface ofa prism beamsplitter can be determined by the following Fresnel formula:$R_{p} = \left\lbrack \frac{\tan\left( {\phi_{1} - \phi_{2}} \right)}{\tan\left( {\phi_{1} + \phi_{2}} \right)} \right\rbrack^{2}$where φ₁ and φ₂ are the incidence and reflection angles, respectively.For example, a prism made of CaF₂ with a refractive index of 1.5 at 193nm can have a preferred range of incidence angles of about 80°-83°relative to the surface normal. Since the beamsplitter does not have acoating on the first surface as in the prior art, the manufacture of thebeamsplitter can be simplified, and the lifetime of the beamsplitterextended.

A beamsplitting prism can be designed, and aligned relative to theincident beam, such that the transmitted portion of the incident beamtraverses the second surface 322 at nearly the Brewster angle, with notheoretical loss. The transmitted beam also will be expanded by a factorof about 3.44 in this example, which can help to reduce the intensity ofthe beam on the folding mirrors. Similarly, the delayed beam can passthrough the third surface 324 and can again be incident onto the firstsurface 320, such that at least a portion of the delayed beam exits fromthe first surface at about 81.6° relative to the normal. The transmittedportion of the delayed beam can be compressed by a ratio of about 3.44,or back to the approximate width of the original optical pulse, and canbe added to the output of the pulse stretching component. Thus, the beamsize is substantially restored upon a complete round-trip through thebeamsplitter and delay line. The process can repeat a number of times,as a portion of the delayed beam will continue to be reflected by thefirst surface as long as the beam contains a sufficient amount ofenergy, providing delayed output pulses with potentially diminishingintensity.

As shown in FIG. 3(a), imaging mirrors 304 and 306 can be used toprovide relay imaging of the beam from the input to the output of thedelay line. In the embodiment of FIG. 3(d), the imaging mirrors areconcave, spherical mirrors with a radius of curvature approximatelyequal to the distance between the mirrors. Alternatively, one can uselenses with flat mirrors as shown in FIG. 3(a). In such an embodiment,the total delay in the delay line can be about 20 nsec, which canrequire about 3.0 meters of distance between mirrors 304 and 306.However, if such space is not available in the laser enclosure, one canuse additional folding mirrors 360, 362 as shown in FIG. 3(d). Such anapproach can be expanded to include additional folding mirrors insimilar fashion, in order to obtain the desired delay in a smallerphysical space. However, additional optics can increase optical lossesand cost, as well as the complexity of alignment. It therefore can bedesirable to use the minimum amount of turning mirrors necessary toobtain the desired path length from the available space. For example, inorder to obtain a 10 m path length in a 3 m space, four mirrors can beused as in FIG. 3(d) to obtain four paths each of approximately 2.5 m inlength.

A beam splitting component 370 in accordance with another embodiment isshown in FIG. 3(e). Here, the beamsplitter 372 is a plano-parallel platewith substantially parallel first and second surfaces 374, 376. Thesecond surface 376 can be coated with an anti-reflective (AR) coating.As discussed with respect to FIGS. 3(a) and 3(b), an incident beam (orpulse) can intercept a first surface 374 of the beamsplitter at apredetermined angle α, which again can be an angle of approximately8.4°, such that the incident ray strikes the first surface at about81.6° from normal to the surface 374. The angle can be selected in orderto obtain the desired reflectance from the first surface, such as asmall angle allowing a reflectance on the order of 30%-35%. Being ableto obtain such a reflectance capability allows the beamsplitter to beused without a reflective, or partially reflective, coating on the firstsurface 374. Since the beamsplitter does not have a coating on the firstsurface as in the prior art, the manufacture of the beamsplitter can besimplified and the lifetime of the beamsplitter extended. Thetransmitted portion of the beam will be expanded as the beam passesthrough the beamsplitter, but will be compressed back to approximatelythe same width upon exiting the second surface 376. A significantadvantage of using small incidence angles is that the spot size formedby the transmitted portion of the beam on the second surface will besignificantly expanded, such that the energy will be spread out over thesurface and any coating on the second surface will encounter a muchlower intensity beam than in other existing systems. This lowerintensity can greatly increase the lifetime of the coating on the secondsurface. The beam should exit the beamsplitter at approximately the sameangle as the angle of incidence α, due to the plano-parallel nature ofthe plate. Upon returning to the beamsplitter, the beam can be incidentupon the second surface 376 at an angle of approximately α. Again, sinceα is a relatively small angle the spot size formed on the second surfacedue to the returning beam will be significantly expanded, as the spotsize increases as the incident angle of the beam moves away from anormal to the surface. This expansion leads to a lower intensity on thecoating and a corresponding longer lifetime.

An anti-reflective coating can be applied to the second surface 376 inorder to prevent reflection of the returning beam. The returning beamagain can be expanded while passing through the beamsplitter, andre-compressed upon exiting the first surface 374. The transmitted beamcan exit the beamsplitter at approximately the same location and alongthe same path as the reflected beam portion. An advantage to such anembodiment is a relatively simpler manufacturing process than an apexprism or dielectrically coated prism, with potential disadvantages of adegradation of the anti-reflective coating and a loss of beam expansioncapability. Additional mirrors may be needed to direct the beam at theappropriate small incidence angles required for such an approach, asexisting systems utilizing partially reflective dielectrically coatedbeamsplitters, for example, typically have an incidence angle ofapproximately 45° or at an angle that is approximately normal to thesurface.

Beam Path Around the Amplifier

In many embodiments of the present invention, additional advantages canbe obtained by using a regenerative power amplifier configuration. In a“regenerative” amplifier configuration, an oscillator beam (or opticalpulse) received from an oscillator, or at least a portion of the beam,makes at least two separate passes through the discharge chamber of theamplifier. The use of at least one additional pass can allow for anincrease in gain, allowing for lower input pulse energy. A multi-passconfiguration also can allow the system to effectively “stretch” theamplified pulse, which can lead to a relaxed requirement of thesynchronization precision and, therefore, greater pulse energyreproducibility. Stretching the beam also can lower the intensity at theoutput optics as discussed elsewhere herein. Optical decoupling can beused between the PA and the MO, as well as between passes of a beamthrough the PA, whereby a high output pulse energy can be obtained withlow energy fluctuations. Further, ASE can be suppressed to less than0.1% of the total laser output.

FIG. 4 shows a top view diagram of an amplifier 402 of a MOPA system,such as that of FIG. 1, which in this embodiment includes a double-passconfiguration, wherein reflecting mirrors 404, 406, 408, 410 are used todirect the beam such that the beam double-passes the amplifier chamber.The beam crosses through the PA at a different angle for each pass, suchas an angle of approximately 0.7° as described in U.S. patentapplication Ser. No. 10/696,979, incorporated by reference above. Afterexiting the MO and passing through a spatial filter and any beam shapingoptics, the beam can be directed through the PA at an angle that issubstantially non-parallel to the long axis of the PA electrodes 116.The beam then can be redirected by a pair of HR mirrors 404, 406 througha spatial filter 412 and any beam shaping optics. The beam only passesthrough the spatial filter 412 after being redirected by HR mirrors 404,406. The beam shaping optics, which can include for example a diverginglens as discussed above, can function in one embodiment to widen thebeam before the beam makes a second pass through the PA. The beam thenmakes a second pass through the PA, at another angle substantiallynon-parallel to the long axis of the PA electrodes before exiting thesystem. The differing angles, and any differing sizes, of the beambetween passes allows the passes to have minimum interference with oneanother, providing a cleaner separation of the “incoming” and “output”beams. In another embodiment, one of the passes of the beam through thePA can be made collinear to the chamber axis, or parallel to the longaxis of the electrodes. The beam can also be the same width for bothpasses, although it can be desirable to widen the beam for the secondpass in order to lower the intensity of the once-amplified beam.

A spatial filter similar to that used in the arrangement of FIG. 4 isshown in more detail in FIGS. 5(a) and 5(b). Spatial filters in generalare well known in the art, and are commonly used for removinghigh-spatial frequency features from beams, as well as combining thefunctions of magnification and imaging. See, for example, J. T. Hunt, P.A. Renard, W. W. Simmons, Applied optics, vol. 16, p. 770 (1977) or J.T. Hunt, J. A. Glaze, W. W. Simmons, P. A. Renard Applied Optics, vol.17, p. 2053 (1978). Spatial filters can consist of two spherical lenses,which form a focal point between the lenses. A pinhole or other aperturecan be placed at the focal point, so that only the highly spatiallycoherent (low divergence) portion of the beam is transmitted. In certainembodiments, cylindrical lenses are used instead of commonly usedspherical lenses, and a slit aperture is used instead of a pinhole.Since the beam is focused into a slit aperture instead of a pinhole, theintensity in the focal point can be reduced by orders of magnitude ascompared to a standard pinhole-based spatial filter. Thus, wear anddamage to the slit aperture can be greatly reduced.

FIGS. 5(a) and 5(b) show embodiments of an exemplary spatial filter 500that can be used in an arrangement such as that shown in FIG. 4. Aspatial filter 500 can consist of two positive lenses 502, 504 and anaperture 506. The positive lenses can be cylindrical or spherical, forexample, and the aperture 506 can comprise a pin-hole or a slit in thesecases, respectively. One difference between the embodiments of FIGS.5(a) and 5(b) is that in FIG. 5(b), the second positive lens 504 ismoved closer to the slit (or pin-hole) than the focal length of thelens. In this case, the output beam will be divergent at the input ofthe final pass of the amplifier. This divergence of the beam can help toreduce the intensity of the beam at the output window of the amplifier,where the intensity is typically the highest in the system. The positivelenses 502, 504 can be placed as close to the tube windows as possible.For example, in the arrangement of FIG. 4 the positive lenses can beplaced between highly reflective mirrors 408 and 410, and between 404and 406 respectively. Aperture 506 can be omitted if no spatialfiltering is required. With the aperture omitted, the spatial filter canact as a relay imaging arrangement, possibly with simultaneousdivergence-increasing functionality.

FIG. 6 shows an amplifier setup in accordance with another embodiment ofthe present invention, wherein an input beam 602 from a masteroscillator passes directly through a partially reflective beamsplitter610 along the beam path before reaching the amplifier chamber 612. Thetransmitted beam 606 is amplified in the chamber and passed through adelay line back to the beamsplitter 610. At the beamsplitter, a portionof the beam is transmitted through the beamsplitter as an output beam608. The remaining portion of the amplified beam is directed backthrough the amplifier chamber 612 and the delay line. The twiceamplified beam portion will again pass back to the beamsplitter 610,where a portion will be directed as an output beam, with a delayrelative to the first output beam related to the length of the circulardelay line, and a portion will again be redirected through the amplifierand delay line. This regenerative amplification process can repeatseveral times, limited only by the presence of sufficient gain in theamplifier, with a portion of the energy being transmitted through thebeamsplitter 610 as output for each pass through the delay line. Anegative lens 614 can be placed along the ring path such that thedivergence of the beam is increased for each subsequent pass through theamplifier, in order to reduce the intensity of the amplified pulseexiting the PA 612. The position of the negative lens in FIG. 6 causesthe beam divergence to be increased only after the first pass of thebeam through the amplifier. This embodiment creates a stretched pulse atthe output of the amplifier, which can reduce the peak intensity of theoutput pulse and minimize damage to a projection lens (not shown) of thesystem. Embodiments can include telescopes and spatial filters such asthose shown in FIGS. 5(a) and 5(b), for example.

In order to input and output a portion of the beam, the beamsplitter 610(here also serving as the input and output mirrors of the ring cavity)can be partially reflecting. The product of the reflectivities of the“input” and “output” partial reflector portions must be small enough tonot cause oscillations in the ring cavity formed by the partialreflectors and reflecting mirrors of the ring cavity. At the same time,the total transmittance through the ring cavity path has to besufficient to create usable input for the second pass through theamplifier. A fundamental assumption in this approach is that the pulseevolution from the noise level in the ring cavity takes longer than thearrival time of the pulse from the oscillator, such that the outputconsists primarily of the amplified main pulse and not ASE. Using thisassumption, the feedback does not necessarily have to be below theoscillation threshold in the absence of the main beam from theoscillator. One potential disadvantage to such an approach is thepotential for an increased level of ASE. Beamsplitter 610, however, cansubstantially prevent ASE from traveling back to the MO.

Examples of a beamsplitter 610 that can be used in the arrangement ofFIG. 6 are shown in FIGS. 7(a) and 7(b). In FIG. 7(a), the beamsplitteris a parallel plate beamsplitter having a dielectric coating 616 on thesecond surface. In FIG. 7(b), the beamsplitter is a prism beamsplitteras discussed above. For this prism setup, however, the beam from the MOis incident on what was previously referred to as the second surface 618and passes into the delay line from what was previously referred to asthe first surface 620. One reason for rotating the prism is to preventany portion of the beam from being reflected from the first surface suchthat the portion passes as output without ever having passed through thepower amplifier. A potential disadvantage to such an orientation,however, is that the beam can be compressed when passing from the firstsurface 620. In this case, it is possible to use beam expanding opticssuch as a negative lens (not shown) to expand the compressed beam inorder to extend the lifetime of the delay line and amplifier optics.Since the beam would be re-expanded when exiting the third surface 622,a beam narrowing element (not shown) can be used before or after thebeam passes through the beam splitter and exits as an output beam.

Such an approach can be used to create a stretched pulse at the outputof the amplifier, which can reduce the peak intensity of the outputpulse of the system. Further, the direction of the beam is always awayfrom the oscillator (not shown) while passing through the amplifier. Avariety of such beam paths can be used with an amplifier of a MOPAsystem, such as are disclosed in pending U.S. patent application Ser.Nos. 10/696,979, filed Oct. 30, 2003, and 10/776,137, filed Feb. 11,2004, each of which is incorporated herein by reference. The use of aring cavity or circular beam path can help to minimize the amount of ASEthat can otherwise disturb the operation of oscillator. The position ofthe reflecting mirrors can be adjusted to increase the beam path in thering cavity to increase the “time window” of the amplification,resulting in the overall gain being less sensitive to the time jitterbetween successive discharges. The ring cavity can include a spatialfilter as described below with respect to FIG. 5. Additionally, orinstead of a spatial filter, a negative lens can be used along the beampath in the cavity. The negative lens can be placed relatively close tothe input of the final pass of the amplifier. The negative lens can be acylindrical or spherical lens. Also, the beam divergence can beincreased as the beam enters the final pass in the amplifier. This canprevent the output window of the amplifier from being exposed to a highpower density beam. Additionally, this can help to compensate foreffects of thermal lensing in the amplifier. A negative lens (sphericalor cylindrical) can be used, or a telescope that is adjusted to a totalnegative optical power and can combine a function of a spatial filter atthe same time.

In order to mitigate the fast degradation of the output window of theamplifier discharge chamber, the cross-sectional area of the amplifiedbeam can be increased in order to reduce the intensity at the windowposition. In order to increase the cross-sectional area, the beam can beexpanded before entering the amplifier, using any of a number of beamexpanding optics or approaches described herein or known or used in theart. A beam 800 also can be aligned at a small angle relative to theelectrodes 802 of the amplification chamber, such as is illustrated inFIG. 8(a), using any of a number of beam directing optics. The beamwidth thus becomes equal to D=d+αL, where D and d are beam width andelectrode width respectively, α is the tilt angle, and L is theelectrode length. One downside of such a solution is a potentialreduction of the overall gain. However, such reduction may be acceptablegiven the benefit of increased optic lifetime.

Another potential downside to the angled beam approach of FIG. 8(a) isthat the beam path typically will require adjustment to obtain theproper small angle relative to the electrode, which can increase thedifficulty of optical alignment. Another approach does not angle thebeam relative to a chamber electrode, but rather splits the electrodeinto portions each of which can be angled with respect to the beam. Forinstance, the electrode portions can be angled with respect to eachother but within the beam path so as to form a V-shape, as shown withthe electrode portions 804, 806 in FIG. 8(b), or a zigzag pattern, suchas shown with the electrode portions 808, 810, 812 in FIG. 8(c). Whilethe main operating principle is similar, the optical alignment issimplified since the beam path becomes symmetrical with respect to thechamber.

Another approach that can be used to further mitigate degradation of theamplifier output window 902 involves moving the output window away fromthe discharge area in the amplifier chamber 900, such as is shown inFIG. 9. Increasing the distance can reduce the likelihood ofcontamination by dust particles generated in the discharge that canotherwise accelerate window decay. Contamination of the window can leadto absorption of the beam at the surface and, thus, generation of heatand failure of the window due to fracture. The output window can beattached to an extender 904, for example, which not only seals theamplifier chamber but also can provide for an increased offset of thewindow from the discharge area. Optical pulse extenders have beendeveloped in order to extend the output pulse after leaving the lasersystem, as described in U.S. Pat. No. 6,389,045 B1 incorporated hereinby reference. The use of such an extender may only be useful for theamplifier chamber, however, as it is standard to minimize the totallength of the optical resonator for the oscillator chamber, while thetotal length is not critical in the amplifier.

Laser System Components

FIG. 10 schematically illustrates an exemplary MOPA excimer or molecularfluorine laser system 1000, which contains various components that canbe used in accordance with embodiments of the present invention as wouldbe understood to one of ordinary skill in the art. The gas dischargelaser system can be a deep ultraviolet (DUV) or vacuum ultraviolet (VUV)laser system, such as an excimer laser system, e.g., ArF, XeCl or KrF,or a molecular fluorine (F₂) laser system for use with a DUV or VUVlithography system. Alternative configurations for laser systems, foruse in such other industrial applications as TFT annealing,photoablation and/or micromachining, e.g., include configurationsunderstood by those skilled in the art as being similar to, and/ormodified from, the system shown in FIG. 10 to meet the requirements ofthat application.

The laser system 1000 includes a MOPA laser component 1002 as describedherein, which includes a master oscillator chamber 1004 and a poweramplifier chamber 1006. Each chamber can have an associated heatexchanger and/or fan for circulating a gas mixture within the chamber.Each chamber can include a plurality of electrodes, such as a pair ofmain discharge electrodes and one or more ionization electrodes orelements, which can be connected with a solid-state pulser module 1008,or with separate pulser modules or circuitry as described elsewhereherein. At least one gas handling module 1010 can have a valveconnection to each chamber, such that halogen, rare and buffer gases,and gas additives, can be injected or filled into the chambers, such asin premixed forms for ArF, XeCl and KrF excimer lasers, as well ashalogen, buffer gases, and any gas additive for an F₂ laser. The gashandling module(s) 1010 can be preferred when the laser system is usedfor microlithography applications, wherein very high energy stability isdesired. Gas handling modules can be optional for a laser system such asa high power XeCl laser. A solid-state pulser module 1008 can be poweredby a high voltage power supply 1012. Alternatively, a thyratron pulsermodule can be used. The oscillator chamber 1004 can be surrounded byoptics modules, forming a resonator. The optics modules can include ahighly reflective resonator reflector in a rear optics module, and apartially reflecting output coupling mirror in a front optics module.The optics modules can be controlled by an optics control module 1014,or can be directly controlled by a computer or processor 1016,particularly when line-narrowing optics are included in one or both ofthe optics modules.

The processor 1016 for laser control can receive various inputs andcontrol various operating parameters of the system. A diagnostic module1018 can receive and measure one or more parameters of a split offportion of the main beam 1020 via optics for deflecting a small portionof the beam toward the module 1018. The diagnostic module can alsoreceive a portion of the beam between the MO and the PA (not shown) inorder to measure one or more parameters of the pulse beforeamplification by the PA. These parameters can include pulse energy,average energy and/or power, and wavelength. The optics for deflecting asmall portion of the beam can include a beam splitter module 1022. Thebeam 1020 exiting the MOPA can be laser output to an imaging system (notshown) and ultimately to a workpiece (also not shown), such as forlithographic applications, and can be output directly to an applicationprocess. Laser control computer 1016 can communicate through aninterface 1024 with a stepper/scanner computer 1026, other control units1028, and/or other, external systems.

The processor or control computer 1016 can receive and process parametervalues, such as may include the pulse shape, energy, ASE, energystability, energy overshoot (for burst mode operation), wavelength,spectral purity, and/or bandwidth, as well as other input or outputparameters of the laser system and/or output beam. The processor canreceive signals corresponding to the wavefront compensation, such asvalues of the bandwidth, and can control wavefront compensation,performed by a wavefront compensation optic in a feedback loop, bysending signals to adjust the pressure(s) and/or curvature(s) ofsurfaces associated with the wavefront compensation optic. The processor1016 also can control the line narrowing module to tune the wavelength,bandwidth, and/or spectral purity, and can control the power supply 1012and pulser module 1008 to control the moving average pulse power orenergy, such that the energy dose at points on a workpiece is stabilizedaround a desired value. The laser control computer 1016 also can controlthe gas handling module 1010, which can include gas supply valvesconnected to various gas sources.

Each laser chamber can contain a laser gas mixture, and can include oneor more ionization electrodes in addition to the pair of main dischargeelectrodes. The main electrodes can be similar to those described atU.S. Pat. No. 6,466,599 B1 (incorporated herein by reference above) forphotolithographic applications.

The solid-state or thyratron pulser module 1008 and high voltage powersupply 1012 can supply electrical energy in compressed electrical pulsesto ionization and/or main electrodes within each laser chamber, in orderto energize the gas mixture. A rear optics module can includeline-narrowing optics for a line narrowed excimer or molecular fluorinelaser as described above, which can be replaced by a high reflectivitymirror or the like in a laser system wherein either line-narrowing isnot desired, or if line narrowing is performed at the front opticsmodule, or a spectral filter external to the resonator is used, or ifthe line-narrowing optics are disposed in front of the HR mirror, fornarrowing the bandwidth of the output beam.

Each laser chamber can be sealed by windows transparent to thewavelengths of the emitted laser radiation 1020. The windows can beBrewster windows, or can be aligned at an angle, such as on the order ofabout 5°, to the optical path of the resonating beam. One of the windowsalso can serve to output couple the beam.

After a portion of the output beam 1020 passes from the MOPA component1002, that output portion can impinge upon a beam splitter module 1022including optics for deflecting a portion of the beam to a diagnosticmodule 1018, or otherwise allowing a small portion of the output beam toreach the diagnostic module 1018, while a main beam portion is allowedto continue as the output beam 1020 of the laser system. The optics caninclude a beamsplitter or otherwise partially reflecting first surfaceoptic, as well as a mirror or beam splitter as a second reflectingoptic. More than one beam splitter and/or HR mirror(s), and/or dichroicmirror(s) can be used to direct portions of the beam to components ofthe diagnostic module 1018. A holographic beam sampler, transmissiongrating, partially transmissive reflection diffraction grating, grism,prism or other refractive, dispersive and/or transmissive optic oroptics also can be used to separate a small beam portion from the mainbeam 1020 for detection at the diagnostic module 1018, while allowingmost of the main beam 1020 to reach an application process directly, viaan imaging system or otherwise.

The output beam 1020 can be transmitted at the beam splitter module,while a reflected beam portion is directed at the diagnostic module1018. Alternatively, the main beam 1020 can be reflected while a smallportion is transmitted to a diagnostic module 1018. The portion of theoutcoupled beam which continues past the beam splitter module can be theoutput beam 1020 of the laser, which can propagate toward an industrialor experimental application such as an imaging system and workpiece forphotolithographic applications.

For a system such as a molecular fluorine laser system or ArF lasersystem, an enclosure (not shown) can be used to seal the beam path ofthe beam 1020 in order to keep the beam path free of photoabsorbingspecies. Smaller enclosures can seal the beam path between each chamber,between the chambers and any optics modules, as well as between the beamsplitter 1022 and the diagnostic module 1018.

The diagnostic module 1018 can include at least one energy detector tomeasure the total energy of the beam portion that corresponds directlyto the energy of the output beam 1020. An optical configuration such asan optical attenuator, plate, coating, or other optic can be formed onor near the detector or beam splitter module 1022, in order to controlthe intensity, spectral distribution, and/or other parameters of theradiation impinging upon the detector.

A wavelength and/or bandwidth detection component can be used with thediagnostic module 1018, the component including, for example, a monitoretalon or grating spectrometer. Other components of the diagnosticmodule can include a pulse shape detector or ASE detector, such as forgas control and/or output beam energy stabilization, or to monitor theamount of amplified spontaneous emission (ASE) within the beam, ensuringthat the ASE remains below a predetermined level. There also can be abeam alignment monitor and/or beam profile monitor.

The processor or control computer 1016 can receive and process valuesfor the pulse shape, energy, ASE, energy stability, energy overshoot forburst mode operation, wavelength, and spectral purity and/or bandwidth,as well as other input or output parameters of the MOPA system andoutput beam. The processor 1016 also can control the line narrowingmodule to tune the wavelength and/or bandwidth or spectral purity, andcan control each power supply 1012 and pulser module 1008 to control themoving average pulse power or energy, such that the energy dose atpoints on the workpiece can be stabilized around a desired value. Inaddition, the computer 1016 can control each gas handling module 1010,which can include gas supply valves connected to various gas sources.Further functions of the processor 1016 can include providing overshootcontrol, stabilizing the energy, and/or monitoring energy input to thedischarge.

The processor 1016 can communicate with each solid-state or thyratronpulser module 1008 and HV power supply 1012, separately or incombination, as well as the gas handling module 1010, the opticsmodules, the diagnostic module 1018, and an interface 1024. Theprocessor 1016 also can control an auxiliary volume, which can beconnected to a vacuum pump (not shown) for releasing gases from eachlaser chamber and for reducing a total pressure in the tube. Thepressure in the tube can also be controlled by controlling the gas flowthrough the ports to and from the additional volume.

A laser gas mixture initially can be filled into each laser chamber in aprocess referred to herein as a “new fill”. In such procedure, a lasertube can be evacuated of laser gases and contaminants, and re-filledwith an ideal gas composition of fresh gas. The gas composition for avery stable excimer or molecular fluorine laser can use helium or neon,or a mixture of helium and neon, as buffer gas(es), depending on thelaser being used. The gas composition can vary between the MO and thePA, as described in U.S. Pat. No. 6,577,663, incorporated herein byreference above.

Total pressure adjustments in the form of releases of gases or reductionof the total pressure within each chamber also can be performed. Totalpressure adjustments can be followed by gas composition adjustments ifnecessary. Total pressure adjustments also can be performed after gasreplenishment actions, and can be performed in combination with smalleradjustments of the driving voltage to the discharge than would be madeif no pressure adjustments were performed in combination.

Line-narrowing features in accordance with various embodiments of alaser system can be used along with the wavefront compensating optic.For an F₂ laser, for example, the optics can be used for selecting theprimary line λ₁ from multiple lines around 157 nm. The optics can beused to provide additional line narrowing and/or to performline-selection. The resonator can include optics for line-selection, aswell as optics for line-narrowing of the selected line. Line-narrowingcan be provided by controlling (i.e., reducing) the total pressure.

Exemplary line-narrowing optics contained in the optics modules and/orbetween chambers can include at least one beam expanding element. Anoptional interferometric device such as an etalon and a diffractiongrating also can be used, which can produce a relatively high degree ofdispersion. A beam expander can include one or more prisms, as well asother beam expanding optics, such as a lens assembly or aconverging/diverging lens pair.

The material used for any dispersive prisms, beam expander prisms,etalons, or other interferometric devices, laser windows, and/or theoutcoupler can be a material that is highly transparent at excimer ormolecular fluorine laser wavelengths, such as 248 nm for the KrF laser,193 nm for the ArF laser and 157 nm for the F₂ laser. The material canbe capable of withstanding long-term exposure to ultraviolet light withminimal degradation effects. Examples of such materials can includeCaF₂, MgF₂, BaF2, LiF, and SrF₂. In some cases fluorine-doped quartz canbe used, while fused silica can be used for the KrF laser. Many opticalsurfaces, particularly those of the prisms, can have an anti-reflectivecoating, such as on one or more optical surfaces of an optic, in orderto minimize reflection losses and prolong optic lifetime.

It should be recognized that a number of variations of theabove-identified embodiments will be obvious to one of ordinary skill inthe art in view of the foregoing description. Accordingly, the inventionis not to be limited by those specific embodiments and methods of thepresent invention shown and described herein. Rather, the scope of theinvention is to be defined by the following claims and theirequivalents.

1. A pulse extender for use with a laser system, comprising: abeamsplitting prism having a planar first surface for reflecting aportion of an incident laser beam and transmitting a portion of theincident beam, the beamsplitting prism further having second and thirdsurfaces forming an apex angle therebetween, the apex angle beingselected such that the transmitted portion of the beam is expanded whenexiting the second surface; and a delay line positioned to receive thetransmitted portion from the beamsplitting prism, the delay lineincluding a plurality of optical elements for directing the transmittedportion along an optical path in order to delay the transmitted portionwith respect to the transmitted portion.
 2. A pulse extender accordingto claim 1, wherein: the optical elements of the delay line direct thetransmitted portion back to the prism upon the transmitted portionexiting the delay line.
 3. A pulse extender according to claim 2,wherein: the transmitted portion exiting the delay line is incident onthe third surface of the beamsplitting prism, whereby at least a portionof the transmitted portion passes through the prism and exits the firstsurface along a beam path of the reflected portion.
 4. A pulse extenderaccording to claim 3, wherein: a portion of the transmitted portionexiting the delay line is reflected from the first surface back into thedelay line.
 5. A pulse extender according to claim 1, wherein: the apexangle is selected so the beamsplitting prism compresses the transmittedbeam portion when the transmitted portion passes through the thirdsurface at an angle approximately equal to the angle at which thetransmitted portion exits the second surface, such that the width of thetransmitted beam exiting the first surface is approximately equal to thewidth of the incident beam.
 6. A pulse extender according to claim 1,wherein: the first surface of the beamsplitting prism is oriented withrespect to the incident beam at an angle whereby about 20% to about 35%of the beam is reflected by the first surface.
 7. A pulse extenderaccording to claim 1, wherein: the first surface of the beamsplittingprism is oriented with respect to the incident beam at an angle betweenabout 80° and 83° from a normal to the first surface.
 8. A pulseextender according to claim 1, wherein: the apex angle and orientationof the beamsplitting prism are selected such that the transmittedportion intersects the second surface at approximately the Brewsterangle, whereby the transmitted portion experiences little loss due toreflection from the second surface.
 9. A pulse extender according toclaim 1, wherein: the apex angle and orientation of the beamsplittingprism are selected such that a subsequent portion of the transmittedbeam portion is reflected by the first surface and exits the secondsurface along a transmitted beam path of the transmitted beam portion.10. A pulse extender according to claim 1, wherein: the apex angle isselected to minimize losses due to reflectance of the transmittedportion at the second surface.
 11. A pulse extender according to claim1, wherein: the apex angle is in the range of from about 90° to about165°.
 12. A pulse extender for use with a laser system, comprising: aparallel plate having planar, parallel first and second surfaces, thesecond surface having an antireflective coating disposed thereon, theplanar first surface angled with respect to an incident laser beam suchthat a portion of the incident beam is reflected from the first surfaceand a portion of the incident beam is transmitted through the first andsecond surfaces, the reflected portion of the incident beam having about25% to about 35% of the intensity of the incident beam; and a delay linepositioned to receive the transmitted portion from the parallel plate,the delay line including a plurality of optical elements for directingthe transmitted portion along an optical path in order to delay thetransmitted portion with respect to the transmitted portion.
 13. A pulseextender according to claim 12, wherein: the angle of the planar firstsurface with respect to the incident beam is between about 80° and 83°from a normal to the first surface.
 14. A pulse extender according toclaim 12, wherein: the optical elements of the delay line direct thetransmitted portion back to the plate upon the transmitted portionexiting the delay line.
 15. A pulse extender according to claim 14,wherein: the transmitted portion exiting the delay line is incident onthe second surface of the parallel plate, whereby at least a portion ofthe transmitted portion passes through the plate and exits the firstsurface along a beam path of the reflected portion.
 16. A pulse extenderaccording to claim 15, wherein: a portion of the transmitted portionexiting the delay line is reflected from the first surface back into thedelay line.